9,161 research outputs found
Lightweight multiple output converter development
A high frequency, multiple output power conditioner was developed and breadboarded using an eight-stage capacitor diode voltage multiplier to provide +1200 Vdc, and a three-stage for -350 Vdc. In addition, two rectifier bridges were capacitively coupled to the eight-stage multiplier to obtain 0.5 and 0.65 a dc constant current outputs referenced to +1200 Vdc. Total power was 120 watts, with an overall efficiency of 85 percent at the 80 kHz operating frequency. All outputs were regulated to three percent or better, with complete short circuit protection. The power conditioner component weight and efficiency were compared to the equivalent four outputs of the 10 kHz conditioner for the 8 cm ion engine. Weight reduction for the four outputs was 557 grams; extrapolated in the same ratio to all nine outputs, it would be 1100 to 1400 grams
Constraining spatial variations of the fine-structure constant in symmetron models
We introduce a methodology to test models with spatial variations of the
fine-structure constant , based on the calculation of the angular power
spectrum of these measurements. This methodology enables comparisons of
observations and theoretical models through their predictions on the statistics
of the variation. Here we apply it to the case of symmetron models. We
find no indications of deviations from the standard behavior, with current data
providing an upper limit to the strength of the symmetron coupling to gravity
() when this is the only free parameter, and not able to
constrain the model when also the symmetry breaking scale factor is
free to vary.Comment: Phys. Lett. B (in press
Linear feedback control of transient energy growth and control performance limitations in subcritical plane Poiseuille flow
Suppression of the transient energy growth in subcritical plane Poiseuille
flow via feedback control is addressed. It is assumed that the time derivative
of any of the velocity components can be imposed at the walls as control input,
and that full-state information is available. We show that it is impossible to
design a linear state-feedback controller that leads to a closed-loop flow
system without transient energy growth.
In a subsequent step, full-state feedback controllers -- directly targeting
the transient growth mechanism -- are designed, using a procedure based on a
Linear Matrix Inequalities approach. The performance of such controllers is
analyzed first in the linear case, where comparison to previously proposed
linear-quadratic optimal controllers is made; further, transition thresholds
are evaluated via Direct Numerical Simulations of the controlled
three-dimensional Poiseuille flow against different initial conditions of
physical interest, employing different velocity components as wall actuation.
The present controllers are effective in increasing the transition thresholds
in closed loop, with varying degree of performance depending on the initial
condition and the actuation component employed
Combined Relativistic and static analysis for all Delta B=2 operators
We analyse matrix elements of Delta B=2 operators by combining QCD results
with the ones obtained in the static limit of HQET. The matching of all the QCD
operators to HQET is made at NLO order. To do that we have to include the
anomalous dimension matrix up to two loops, both in QCD and HQET, and the one
loop matching for all the Delta B=2 operators. The matrix elements of these
operators are relevant for the prediction of the B-\bar B mixing, B_s meson
width difference and supersymmetric effects in Delta B=2 transitions.Comment: 3 pages, 1 figure. Lattice2001(heavyquark
Generation of different Bell states within the SPDC phase-matching bandwidth
We study the frequency-angular lineshape for a phase-matched nonlinear
process producing entangled states and show that there is a continuous variety
of maximally-entangled states generated for different mismatch values within
the natural bandwidth. Detailed considerations are made for two specific
methods of polarization entanglement preparation, based on type-II spontaneous
parametric down-conversion (SPDC) and on SPDC in two subsequent type-I crystals
producing orthogonally polarized photon pairs. It turns out that different Bell
states are produced at the center of the SPDC line and on its slopes,
corresponding to about half-maximum intensity level. These Bell states can be
filtered out by either frequency selection or angular selection, or both. Our
theoretical calculations are confirmed by a series of experiments, performed
for the two above-mentioned schemes of producing polarization-entangled photon
pairs and with two kinds of measurements: frequency-selective and
angular-selective.Comment: submitted for publicatio
Non-perturbative renormalisation of four fermion operators and B-bar B mixing with Wilson fermions
We present new results for the renormalisation and subtraction constants for
the four fermion Delta F=2 operators, computed non-perturbatively in the RI-MOM
scheme (in the Landau gauge). From our preliminary analysis of the lattice data
at beta=6.45, for the B-bar B mixing bag-parameter we obtain B_B^{RGI} =
1.46(7)(1).Comment: 3 pages (4 figures), Lattice2002(heavyquark
Low-Metallicity Gas Clouds in a Galaxy Proto-Cluster at Redshift 2.38
We present high resolution spectroscopy of a QSO whose sight-line passes
through the halo of a pair of elliptical galaxies at redshift 2.38. This pair
of galaxies probably lies at the center of a galaxy proto-cluster, and is
embedded in a luminous extended Ly-alpha nebula.
The QSO sight-line intersects two small gas clouds within this halo. These
clouds have properties similar to those of high velocity clouds (HVCs) seen in
the halo of the Milky Way. The gas is in a cool (< 2 x 10^4 K) and at least 20%
neutral phase, with metallicities in the range -3.0 < [Fe/H] < -1.1 and neutral
hydrogen column densities of ~10^19.5 /cm^2.
The origin of these clouds is unclear. The presence of low metallicity gas
within this possible proto-cluster implies either that the intra-cluster medium
has not been enriched with metals at this redshift, or the clouds are embedded
within a hot, ionized, metal-rich gas phase.Comment: Accepted to appear in ApJ Letter
Axially open nonradiative structures: an example of single-mode resonator based on the sample holder
The concept of nonradiative dielectric resonator is generalized in order to
include axially open configurations having rotational invariance. The resulting
additional nonradiative conditions are established for the different resonance
modes on the basis of their azimuthal modal index. An approximate chart of the
allowed dielectric and geometrical parameters for the TE011 mode is given. A
practical realization of the proposed device based on commercial fused quartz
tubes is demonstrated at millimeter wavelengths, together with simple
excitation and tuning mechanisms. The observed resonances are characterized in
their basic parameters, as well as in the field distribution by means of a
finite element method. The predictions of the theoretical analysis are well
confirmed, both in the general behaviour and in the expected quality factors.
The resulting device, in which the sample holder acts itself as single-mode
resonating element, combines an extreme ease of realization with
state-of-the-art performances. The general benefits of the proposed open
single-mode resonators are finally discussed.Comment: 18 pages, 10 figure
Polarized and Unpolarized Nucleon Structure Functions from Lattice QCD
We report on a high statistics quenched lattice QCD calculation of the
deep-inelastic structure functions , , and of the proton
and neutron. The theoretical basis for the calculation is the operator product
expansion. We consider the moments of the leading twist operators up to spin
four. Using Wilson fermions the calculation is done for three values of
, and we perform the extrapolation to the chiral limit. The
renormalization constants, which lead us from lattice to continuum operators,
are calculated in perturbation theory to one loop order.Comment: 17 pages, uuencoded postscript file. Renormalization constant of
now include
Evolution of the fine-structure constant in runaway dilaton models
We study the detailed evolution of the fine-structure constant in
the string-inspired runaway dilaton class of models of Damour, Piazza and
Veneziano. We provide constraints on this scenario using the most recent
measurements and discuss ways to distinguish it from alternative
models for varying . For model parameters which saturate bounds from
current observations, the redshift drift signal can differ considerably from
that of the canonical CDM paradigm at high redshifts. Measurements of
this signal by the forthcoming European Extremely Large Telescope (E-ELT),
together with more sensitive measurements, will thus dramatically
constrain these scenarios.Comment: 11 pages, 4 figure
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